Holding device for wafers

ABSTRACT

The invention relates to a holding device for wafers in an arrangement for wafer inspection, comprising two grippers ( 11, 12 ), each of which, in the closed state of the holding device ( 8 ), encloses a subsection of the wafer circumference and which are connected to a drive device ( 27 ) and, when the latter is driven, the grippers ( 11, 12 ) move away from each other for the purpose of opening the holding device ( 9 ) and move toward each other for the purpose of closing the holding device ( 8 ), and a holding arm ( 13 ), on which the two grippers ( 11, 12 ) are pivotably mounted.  
     In this case the holding arm ( 13 ) is mounted such that it can rotate about an axis (A) which lies substantially in the plane covered by the wafer (W), so that after a rotation through 180° about the axis (A), a wafer (W) held between the grippers ( 11, 12 ) has been turned.

[0001] The invention relates to a holding device for wafers in anarrangement for wafer inspection, comprising two grippers, each ofwhich, in the closed state of the holding device, encloses a subsectionof the wafer circumference and which are connected to a drive deviceand, when the latter is driven, the grippers move away from each otherfor the purpose of opening the holding device and move toward each otherfor the purpose of closing the holding device, and comprising a holdingarm on which the two grippers are pivotably mounted.

[0002] A holding device of this type is disclosed by U.S. Pat. No.5,700,046 A. The holding device described there is mounted such that itcan move and is used for the purpose of holding wafers as they aretransferred, for example, from a pick-up station to an inspectionstation. For this purpose, the holding device can move along ahorizontal axis, can be moved up and down and pivoted about an axis atright angles to the transport direction. The transport direction and thedirection of the main extent of the wafer always lie in one plane inthis case, that is to say the alignment of the two opposite surfaces ofthe wafer with respect to the transport direction is maintained duringthe transfer.

[0003] During the transfer, the wafer is enclosed in the manner of anarc by the grippers. In the process, the wafer bears only against theintermediate pieces provided on the grippers. In the holding devicedescribed in U.S. Pat. No. 5,700,046 A, these are constructed in such away that the wafer is held flatly only on its unstructured underside orrear side and at points on its circumferential side edge. Theintermediate pieces are L-shaped for this purpose. In this case, thewafer rests with its underside or rear side on horizontal legs of theintermediate pieces and is kept centered by means of the vertical legs,which bear against the side edge of the wafer.

[0004] U.S. Pat. No. 5,807,062 A discloses an inspection arrangement inwhich a wafer passes through various inspection stations. Here, inaddition to a microinspection station, in which a microscopicexamination of the wafer is carried out, a macroinspection station isalso provided, in which the wafer is assessed directly visually by aninspector. In this case, it is primarily large-area faults which may bedetected, such as scratches or color deviations. Moreover, contaminants,for example dust particles, adhering to the wafer can be detected.

[0005] However, since in the inspection arrangement described in U.S.Pat. No. 5,807,062 A the transport direction and the direction for themain extent of the wafer always likewise lie in one plane, that is tosay the alignment of the structured upper side and the nonstructuredunderside of the wafer with respect to the transport direction ismaintained during the transfer, and turning of the wafer is not possiblewith this holding device, it is, however, possible to inspect only thatsurface of the wafer which faces the inspector or the microscopeobjective. This is generally the structured upper side of the wafer.From time to time, however, there is an interest in assessing theunderside or rear side of the wafer as well.

[0006] On this basis, the invention is based on the object of providinga device which permits viewing both of the structured and of thenonstructured surface of a wafer.

[0007] To this end, a holding device of the type mentioned at thebeginning is proposed in which the holding arm is mounted such that itcan rotate about an axis A, which lies substantially in the planecovered by the wafer, so that after a rotation through 180° about theaxis, a wafer held between the grippers has been turned.

[0008] The axis A should advantageously run approximately centrallybetween two pivoting bearings for the grippers.

[0009] With the aid of the holding device according to the invention, awafer can be held securely and turned in any desired transport positionof the holding device, that is to say in an inspection station or evenhalfway between two stations, so that it is possible to inspect both thefront side and the rear side from a defined viewing station. Since thewafer is held by the grippers only at a side edge, the view of both itssurfaces is not restricted or only insignificantly restricted.

[0010] The holding device is preferably used in an arrangement for thevisual inspection of wafers. This arrangement comprises a transferstation, a macroinspection station and a microinspection station andalso a transfer device for transporting a wafer between the individualstations. In this case, the holding device is arranged in themacroinspection station.

[0011] Therefore, during a passage of a wafer through the inspectionarrangement, both its structured front side and its rear side can beassessed macroscopically.

[0012] In this case, it is also necessary to take account of the factthat the wafer must not be put down on its structured front side, sincethis could lead to damage which would make the wafer unusable. With theholding device, such impairment of the front side during the inspectionof the wafer is avoided.

[0013] In an advantageous refinement of the invention, the holding armis mounted such that it can pivot about a further axis B, which runssubstantially tangentially with respect to the wafer circumference andintersects the axis A at least approximately at right angles. This makesit possible to pivot the wafer out of its position before turning it.This is advantageous in particular when components of the waferinspection device could collide with the wafer or the grippers duringturning.

[0014] The axis A should advantageously run approximately centrallybetween two pivoting bearings for the grippers.

[0015] Such a collision may also be avoided by the holding arm beingmounted such that it can be displaced in a rectilinear guide, thedisplacement direction C intercepting the axes A and B substantially atright angles. It is also possible in this way to lift the wafer out ofits original transport plane to such an extent that a collision duringturning is avoided.

[0016] By means of a combination of the aforementioned possiblemovements, the movement travel and the pivoting angle may be reduced, asa result of which the time for turning the wafer can be reduced. Themovements, that is to say the translation movement and the pivotingmovements of the holding arm, are preferably carried out at the sametime.

[0017] The possibility of a well coordinated movement sequence resultsif in each case separately driveable drive devices are provided forpivoting the grippers, for turning the wafer by means of rotation aboutthe axis A, for pivoting the holding arm about the axis B and fordisplacing the holding arm in the displacement direction C.

[0018] In a particularly advantageous refinement of the invention, thegrippers each have, on their portions facing the wafer circumference,intermediate pieces, of which at least one is shaped geometrically suchthat the wafer is held only by contact at its circumferential edges. Inthis way, firstly secure holding of the wafer is ensured, secondly anycontact with the front side is avoided. Furthermore, the impairment ofthe visible surface in particular on the rear side of the wafer as well,remains very low. As a result of the very small contact areas, the riskof transferring particles is also reduced.

[0019] In principle, it is possible to construct individual intermediatepieces in such a way that these come into engagement with onecircumferential edge and other intermediate pieces come into engagementonly with the other circumferential edge of the wafer. However, at leastone intermediate piece is preferably provided, which, during gripping,bears against both circumferential edges of the wafer. Such anintermediate piece can, for example, be open in a V shape toward thewafer, the circumferential edges of a wafer that is held resting in theV profile.

[0020] Furthermore, it is possible to construct such an intermediatepiece as a roller, which is mounted eccentrically on a gripper and canbe fixed in a selected angular position. This makes it possible, in aparticularly simple manner, to perform an adjustment of the positionbetween the wafers and the grippers.

[0021] As a result of the resilient support of at least one intermediatepiece on a gripper, firstly secure gripping is ensured, secondly thecontraction forces at the points of contact can be managed well, so thatdamage can be avoided with great certainty.

[0022] The resilient support can be provided, for example, via a leafspring, the intermediate piece being attached to the associated gripperby a leaf spring. This has the advantage that the holding force of thespring is not impaired during turning. Instead, the position of thewafer relative to the grippers is maintained during turning.

[0023] Furthermore, it is also possible to prestress the grippermovements themselves resiliently, in order in this way to keep the riskof damage to the circumferential edges of the wafer low, in particularwhen closing the grippers.

[0024] In a further advantageous refinement of the invention, at leastone stop is provided between the grippers and the holding arm, whichlimits the pivoting angle during the gripper movement. The security ofthe holding device is in particular improved thereby.

[0025] In order to ensure secure holding of a wafer in the holdingdevice, in particular even in the event of failure of the operatingvoltage, a tension spring is provided between the grippers, by means ofwhich the wafer is held between the grippers under elastic tension, evenwithout any drive force.

[0026] In a further advantageous refinement of the invention, a sensorfor monitoring the presence of a wafer between grippers is provided onat least one gripper. This is advantageous in particular for anautomated operating mode of the holding device, in which a wafer isgripped automatically, subsequently turned, then rotated back into itsoriginal position and finally released again.

[0027] The invention will be explained in more detail below using anexemplary embodiment illustrated in the drawing, in which:

[0028]FIG. 1 shows a schematic representation of an inspectionarrangement having a holding device for turning a wafer,

[0029]FIG. 2 shows a three-dimensional view of the holding device,

[0030]FIG. 3 shows a side view of the holding device in a first positionbefore gripping a wafer,

[0031]FIG. 4 shows a side view of the holding device in a secondposition when gripping a wafer,

[0032]FIG. 5 shows a side view of the holding device in a third positionimmediately before turning the wafer, in which position the wafer hasalready been lifted upward and pivoted,

[0033]FIG. 6 shows a detailed view of the gripper in the region of thewafer,

[0034]FIG. 7 shows a detailed view of an intermediate piece,

[0035]FIG. 8 shows a detailed view of a resilient intermediate piece,

[0036]FIG. 9 shows a detailed view of a sensor, and

[0037]FIG. 10 shows a detailed view of a stop for the grippers.

[0038] The first exemplary embodiment shows, in FIG. 1, an inspectionarrangement 1 with which two-dimensional substrates such as wafers W canbe examined both directly visually and also microscopically. Inparticular, by using the inspection arrangement 1, wafers W assessed asfaulty during fabrication and separated out may be examined moreclosely, in order as a result to draw conclusions about the cause of thefault. However, the inspection arrangement can also be used for otherexaminations.

[0039] In the exemplary embodiment illustrated, the inspectionarrangement 1 comprises three stations, at which different tasks orpreparatory activities are carried out. These three stations, namely atransfer station 2, a macroinspection station 3 and a microinspectionstation 4 are arranged in a common housing 5, which is closed off to theoutside. The individual wafers W are transported between the individualstations 2, 3 and 4 by means of a transfer device 6. Here, the transferdevice 6 comprises a star-shaped holder, which rotates about a centralaxis Z and is stopped at the individual stations.

[0040] In the transfer station 2, a wafer W is loaded into theinspection arrangement 1 by means of a transfer device 7, indicated, andis put down on the transfer device 6. From there, with a rotation of thetransfer device 6 through 120°, the wafer W passes into themacroinspection station 3. In the latter, an assessment is carried outby means of an inspector P standing or sitting beside the inspectionarrangement 1.

[0041] In the transfer state, the structure front side of the wafer Wpoints upward, while the unstructured rear side of the wafer W rests onthe transfer device 6. After the front side has been viewed, the wafer Wis turned, so that the inspector P can also inspect its rear side. Forthis purpose, a holding device 8 is provided in the macroinspectionstation 3 and firstly grips the wafer W, lifts it upward off thetransfer device 6 and then turns it. Following inspection of the rearside of the wafer, the wafer W is firstly turned back and then put downwith the rear side on the transfer device 6 again.

[0042] By indexing the transfer device 6 by 120° again, the wafer Wpasses into the microinspection station 4. Here, the wafer W can beexamined in detail, section by section, with a microscope 9. Themicroscope 9 comprises a moveable table 10, on which the wafer W to beinspected is put down temporarily for the purpose of inspection. Afterthe inspection, the wafer W is transported onward to the transferstation 2, in which the wafer W is removed by means of the transferdevice 7. Suitable transfer devices are known to those skilled in theart and therefore do not require any specific explanation. For example,use can be made here of a device as described in U.S. Pat. No. 5,700,046A.

[0043] In the following text, the holding device 8 according to theinvention for turning a wafer W, arranged in the macroinspection station3, is to be explained in more detail.

[0044] The holding device 8, which is illustrated in detail in FIG. 2,comprises two arc-shaped grippers 11 and 12 which, in terms of theirshape, are matched to the diameter of the wafers W to be gripped andenclose about three quarters of the wafer at its side edge. The twogrippers 11 and 12 are mounted on a holding arm 13 and can be moved withrespect to each other. In order to open them, the grippers 11 and 12 arepivoted away from each other, so that the inner sides of the gripperscan be guided to the level of the wafer edge. In order to grip a waferW, the holding device is closed, that is to say the grippers 11 and 12are moved toward each other until the latter come into contact with thewafer edge by means of the intermediate pieces 14 and 15.

[0045] The intermediate pieces 14 and 15 are in this case constructed insuch a way that, when they grip a wafer W, they bear only against thecircumferential edges at the wafer circumference, as shown in FIG. 7. Bythis means, firstly secure gripping is ensured, secondly any contactwith the structured front side and also the rear side of the wafer W isavoided. Moreover, the area of contact between the contraction elements14 and 15 and the wafer W remains small, so that as few particles aspossible are loosened or transferred when gripping and releasing thewafer W.

[0046] In the selected exemplary embodiment, a total of four uniformlydistributed intermediate pieces 14 are permanently arranged on thegrippers 11 and 12, each being designed in the manner of a roller. Whenviewed in a plane at right angles to the gripping plane, this rollerpossesses a wedge-shaped profile which tapers toward the roller center.The two mutually opposite wedge surfaces 16 and 17 in each case bearagainst a circumferential edge of the wafer W when gripping, and in thisway also effect centering of the wafer W in the gripping plane.

[0047] The permanently arranged intermediate pieces 10 are in each casemounted eccentrically on the associated gripper 11 or 12. As a result ofa rotation, in this way the position of a gripped wafer W relative tothe grippers 11 and 12 can be set. After an adjustment has been carriedout, the intermediate pieces 14 are fixed in the selected angularposition.

[0048] Apart from the stationary intermediate pieces 14, a resilientintermediate piece 15 is also provided, which is shown in detail in FIG.8. In terms of its shape, this intermediate piece 15 correspondssubstantially to half an intermediate piece 14, so that the resilientlymounted intermediate piece 15 can also bear with two wedge surfaces 16and 17 against the circumferential edges of a wafer W. The intermediatepiece 15 is fixed to a leaf spring 18 which, for its part, is in turnfixed to the inner flank of the gripper 11 which points toward the sideedge of the wafer W. The leaf spring 18 can be deflected elastically inthe gripping plane but, on the other hand, is stiff in a direction atright angles to this. This radial springing permits particularly gentlegripping of a wafer W.

[0049] Furthermore, in each case a sensor 19 is fitted to the grippers11 and 12, with which the presence of a wafer W between the grippers 11and 12 can be determined. The signal generated by the sensors 19 isevaluated in a control device, not specifically illustrated, in order topermit automated operation of the holding device 8. For example, beforea wafer is turned, it is possible to determine whether said wafer hasbeen gripped properly. Moreover, when the wafer W is put down on thetransfer device 6 again, it is possible to determine whether, before thegrippers 11 and 12 are removed from the wafer W, the latter has alsoactually been released. The sensors 19 are constructed and arrangedaccordingly. In the exemplary embodiment illustrated, they are locatedon the inner flanks of the grippers 11 and 12.

[0050] In order to open and close the grippers 11 and 12, they arepivotably mounted on the holding arm 13. The pivoting movement of thegrippers 11 and 12 is limited by a stop 20 fitted to the gripper 11.Said stop is illustrated in detail in FIG. 10, which shows a closedposition of the grippers 11 and 12. In this closed position, the stop 20strikes a stop surface 21 on the holding arm 13.

[0051] Arranged beside the stop 20 is a tension spring, which isconnected to both grippers 11 and 12 in order to pull the latter intothe closed position. This ensures that a wafer W always remains heldsecurely between the wafers 11 and 12. The opening and closing arecarried out by means of a drive device, which is coupled to the grippers11 and 12.

[0052] In order to turn the wafer W, a further drive device is provided,which permits rotation of the wafer through 180° about an axis A.Following a rotation about this turning axis A, the wafer W issubstantially in the initial position, but the front side and the rearside have been interchanged. As FIG. 2 shows, the turning axis W runsthrough the holding arm 13, parallel to or through the gripping plane,and preferably also through the mid-axis M of the wafer W.

[0053] The holding arm 13 is in turn pivotably mounted on a bracket 23.The pivot axis B in this case runs transversely with respect to theturning axis A. In the exemplary embodiment illustrated, the pivot axisB runs horizontally and outside a wafer W gripped between the grippers11 and 12.

[0054] The bracket 23 is for its part supported on a base part 24 andguided linearly with respect to the latter in the displacement directionC. The linear guide 25 here permits an upward and downward movement ofthe brackets 23 and therefore, at the same time, of the holding arm 13and of the grippers 11 and 12.

[0055] Each of the individual movements, that is to say the opening andclosing of the grippers 11 and 12, the turning of the holding arm 13with the grippers 11 and 12 about the turning axis A, the pivoting ofthe holding arm 13 about the pivot axis B and also the upward anddownward movement of the bracket 23 along the displacement direction C,are carried out via a separately controllable drive device. In thiscase, actuating or stepper motors can be provided, so that theindividual movements can be carried out completely independently of oneanother. The individual drive devices are designated by the designations26 to 29 in FIGS. 2 to 5.

[0056] In the following text, the operating mode of the holding device 8is now to be explained in more detail using FIG. 3 to FIG. 5. In thiscase, FIG. 3 shows an initial position, in which a wafer W is conveyedinto the macroinspection station 3 in a transfer plane T by means of thetransfer device 6. In this case, the grippers 11 and 12 are located in aposition underneath the transfer plane T. The gripping plane of thegrippers 11 and 12 is aligned approximately parallel to the transferplane T. In order to grip the wafer W, first of all the drive device 26is actuated in order to move the bracket 23 along the displacementdirection C. Said drive device 26 is constructed as a linear motor 26here. At the same time, the grippers 11 and 12 are opened as a result ofactuation of the associated drive device 27, so that the intermediatepieces 14 and 15 do not collide with the wafer W as they move upward.

[0057] The upward movement along the displacement direction C is stoppedwhen the gripping plane coincides with the transfer plane T. By means ofrenewed actuation of the drive device 27, an electric motor here, thegrippers 11 and 12 are moved into the closed position, whereupon theintermediate pieces 14 and 15 are brought into contact with the wafer Wand in the process engage around its side edge. This state isillustrated in FIG. 4. When proper gripping of the wafer W is signalledby means of the sensors 19, the drive device 26 is actuated again inorder to lift the wafer W off the transfer device 6 and to move itfurther upward. At the same time, or else with a small time delay, theholding arm 13 is pivoted about the axis B with respect to the bracket23, as indicated in FIG. 5. For this purpose, the associated drivedevice 28, which is again constructed as an electric motor, is actuated.The wafer W is moved upward and pivoted about the axis B until thegrippers 11 and 12 with the wafer W can be rotated without hindranceabout the turning axis A, without colliding with the transfer device 6in the process.

[0058] As a result of the possibility of pivoting about the pivot axisB, the movement travel along the displacement direction C can be keptrelatively small. The simultaneity of the two movements makes itpossible to quickly reach the position illustrated in FIG. 5 in whichthe actual turning can then be performed.

[0059] The assessment of the rear side of the wafer W can be performedby means of an appropriate alignment of the holding device 8 in thispivoted position, from a viewing space beside the inspectionarrangement. For this purpose, the drive device 29 is actuated in orderto rotate about the axis A.

[0060] By means of a combined pivoting movement about the axes A and Bby small angles, a tumbling movement with the wafer W can be carried outin the position illustrated in FIG. 5 and, given appropriateillumination of the wafer W, improves the visual detectability of faultson the wafer W and also dust particles adhering thereto. This tumblingmovement can be carried out both for viewing the structured front sideand for viewing the rear side.

[0061] However, it is also conceivable to move the wafer W back into aposition corresponding to FIG. 4, but said wafer is stopped shortlyabove the transfer device 6, in order not to damage the structured frontside of the wafer W. In this way, virtually identical observationconditions can be implemented for the inspection of the front and rearsides. However, this is somewhat more time-consuming than the inspectionin the position illustrated in FIG. 5, since the pivoting and liftingmovements with respect to the axis B and the displacement direction Cthen have to be carried out twice.

[0062] After the wafer W has been turned back into the positionillustrated in FIG. 5, it is then put down with its rear side on thetransfer device 6, for which purpose the holding device 8 is moved intothe position illustrated in FIG. 4. The wafer is released by opening thegrippers 11 and 12. This is also detected by means of the sensors 19. Bymeans of lowering the bracket 23 further, the initial positionillustrated in FIG. 3 is then reached, in which the grippers 11 and 12are pulled back from the transfer plane T and are closed in order torelieve the load on the associated drive device 27.

[0063] The holding device explained permits rapid and efficientmicroscopic inspection of the front and rear sides of a wafer in oneoperation. As a result of the arrangement in the macroinspectionstation, the scope of the inspection of an inspection arrangement isincreased.

LIST OF DESIGNATIONS

[0064]1 Inspection arrangement

[0065]2 Transfer station

[0066]3 Macroinspection station

[0067]4 Microinspection station

[0068]5 Housing

[0069]6 Transfer device

[0070]7 Transfer device

[0071]8 Holding device

[0072]9 Microscope

[0073]10 Table

[0074]11 Gripper

[0075]12 Gripper

[0076]13 Holding arm

[0077]14 Intermediate piece

[0078]15 Resilient intermediate piece

[0079]16 Wedge surface

[0080]17 Wedge surface

[0081]18 Leaf spring

[0082]19 Sensor

[0083]20 Stop

[0084]21 Stop surface

[0085]22 Tension spring

[0086]23 Bracket

[0087]24 Base part

[0088]25 Linear guide

[0089]26, 27, 28, 29 Drive devices

[0090] W Wafer

[0091] R Axis of rotation

[0092] A, B Axes

[0093] C Displacement direction

[0094] M Mid-axis

1. A holding device for wafers in an arrangement for wafer inspection,comprising: two grippers (11, 12), each of which, in the closed state ofthe holding device (8), encloses a subsection of the wafer circumferenceand which are connected to a drive device (27) and, when the latter isdriven, the grippers (11, 12) move away from each other for the purposeof opening the holding device (9) and move toward each other for thepurpose of closing the holding device (8), and a holding arm (13), onwhich the two grippers (11, 12) are pivotably mounted, characterized inthat the holding arm (13) is mounted such that it can rotate about anaxis (A) which lies substantially in the plane covered by the wafer (W),so that after a rotation through 180° about the axis (A), a wafer (W)held between the grippers (11, 12) has been turned.
 2. The holdingdevice as claimed in claim 1, characterized in that the holding arm (13)is mounted such that it can pivot about a further axis (B), which runssubstantially tangentially with respect to the wafer circumference andintersects the axis (A) at least approximately at right angles.
 3. Theholding device as claimed in claim 1 or 2, characterized in that theholding arm (13) is mounted such that it can be displaced in arectilinear guide, the displacement direction (C) intercepting the axes(A, B) substantially at right angles.
 4. The holding device as claimedin claim 3, characterized in that the axis (A) runs approximatelycentrally between two pivoting bearings for the grippers (11, 12). 5.The holding device as claimed in one of the aforementioned claims,characterized in that in each case separate drive devices (26, 27, 28,29) are provided for pivoting the grippers (11, 12), for turning thewafer (W) by means of rotation about the axis (A), for pivoting theholding arm (13) about the axis (B) and for displacing the holding arm(13) in the displacement direction (C).
 6. The holding device as claimedin one of the aforementioned claims, characterized in that the grippers(11, 12) each have, on their portions facing the wafer circumference,intermediate pieces (14, 15), of which at least one is shapedgeometrically such that the wafer (W) is held only by contact at itscircumferential edges.
 7. The holding device as claimed in claim 6,characterized in that at least one intermediate piece (14, 15) isprovided which, during gripping, bears against both circumferentialedges of the wafer (W).
 8. The holding device as claimed in claim 7,characterized in that the at least one intermediate piece (14, 15) isopen in a V shape toward the wafer, and the circumferential edges of awafer (W) that is held rest in the V profile.
 9. The holding device asclaimed in one of claims 6 to 8, characterized in that at least one ofthe intermediate pieces (14) is constructed as a roller, which ismounted eccentrically on a gripper (11, 12) and can be fixed in aselected angular position.
 10. The holding device as claimed in one ofclaims 6 to 9, characterized in that at least one of the intermediatepieces (15) is supported resiliently on the gripper (11, 12).
 11. Theholding device as claimed in claim 10, characterized in that theresiliently supported intermediate piece (15) is attached to theassociated gripper (11, 12) by a leaf spring (18).
 12. The holdingdevice as claimed in one of the aforementioned claims, characterized inthat the grippers (11, 12) are resiliently prestressed in their pivotingbearings.
 13. The holding device as claimed in one of the aforementionedclaims, characterized in that stops (20) are provided between thegrippers (11, 12) and the holding arm (13), which limit the pivotingangle during the gripper movement.
 14. The holding device as claimed inone of the aforementioned claims, characterized in that the grippers(11, 12) are connected to each other by a tension spring (22), by meansof which the wafer (W) is held between the grippers (11, 12) underelastic tension.
 15. The holding device as claimed in one of theaforementioned claims, characterized in that there is a sensor (19) onat least on [sic] one gripper (11, 12) for monitoring the presence of awafer (W) between the grippers (11, 12).